Attention is directed to a paper by the applicant entitled "Laser Deflection Using The Photorefractive Effect" published by the Massachusetts Institute of Technology Lincoln Laboratory (Jan. 12, 1981), wherein some of the applicant's work is described, hereby incorporated by reference.
Interest in the use of lasers for radar, optical display systems, optical memory and retrieval systems has prompted a search into more effective and efficient beamsteering devices. Such devices can be divided into two main categories, those which provide analog deflection and those which provide digital deflection. The first type of analog deflection is provided by source motion. A point source of laser light placed in the focal plane of a lens will produce a collimated laser beam which varies in direction as the point source is moved. These devices have the advantage of being easily controlled and two dimensional deflection can be produced. These devices, however, are limited by the fact that the laser source be compact enough to function at a point source. This limits the laser device to low-power. The second type of analog device overcomes the point source limitation by tilting the entire wavefront from a lasing medium using a phased array or grating. A fundamental difficulty in this technique is that the multiple diffraction orders produced by such phase gratings limit the efficiency. of these devices to about 40%. This limitation on efficiency can be overcome if Bragg's law is satisfied using a thick grating. Acousto-optic devices are presently in use for a number of commercial purposes employing such thick gratings, for example, U.S. Pat. No. 3,860,752 issued to Adler et al on Jan. 14, 1975 disclosing an acousto-optic beam-steering device.
The second main type of beamsteering device can be characterized by digital deflection rather than analog deflection. There are at least three different types of digital deflectors. The first operates on the principle of polarization modulation, and makes use of a polarizing beamsplitter to direct beams into one of two directions depending upon the polarization of the beam. Such schemes are inherently binary, and all presently known methods of modulating and separating different polarizations have limited fields-of-view. However, these devices can operate at very high speeds. The second method of digital deflection makes use of the principle of total internal reflection. These devices operate by changing the critical angle of an interface slightly, thus either transmitting or reflecting an incident beam. Again, these switches are inherently binary. Since the beam must be incident on an interface near the critical angle a substrate much larger than the beam diameter is required. This also tends to limit the field-of-view if the critical angle is large. The third method of digital switching is based on the use of interferometers. By changing the path length difference in an interferometer, the resulting constructive or destructive interference can be used to switch a beam from one output to another. The number of potential outputs is equal to the finesse of the interferometer, which in the case of a Fabry-Perot etalon can be larger than two. Furthermore, if the path length change compensates for the angle of the beam going through the device, the limitation on the field-of-view can be relaxed. All the above digital schemes can be cascaded in order to produce larger numbers of beam positions, however, when large numbers of beam positions are required the transmission of each stage must be very near unity in order to avoid unacceptable losses in the beamsteering system.
Aside from beamsteering devices, it is generally known that the photorefractive effect is a bulk phenomenon occurring in certain single crystal materials such as lithium niobate, zinc selenide, bismuth silicon oxide and bismuth germanium oxide. These materials have both photoconductive and electro/optic properties. Briefly, the mechanism involves the creation of photoelectrons by incident short wavelength radiation (.perspectiveto.5000 .ANG.). These electrons move under the influence of an applied bias voltage to regions of low intensity in the crystal, where they are trapped. This creates a strong internal electric field which modulates the index of refraction of the material through a transverse electro-optic effect. I am not aware of any prior art device employing photorefractive materials to steer a laser beam in a random access fashion.
There exists a need in connection with laser radar, optical displays, memory and retrieval systems for a beamsteering device with high efficiency and a wide field-of-view, which can produce up to several thousand beam positions or more with rapid steering (i.e. less than one millisec). It would also be preferrable to have a beamsteering device for a high power laser beam which can be controlled by a relatively low power, agile, control beam, thus permitting all-optical steering.